(1) Field of the Invention
The present invention relates to a load condition determining apparatus adapted to accurately and effectively detect electrical load conditions of a load consisting of, for instance, various lamps equipped in vehicles.
(2) Description of the Prior Art
It is important to supervise or monitor operating conditions of different lights and lamps, such as headlights, tail lamps, brake lamps, to detect any abnormal conditions such as any short circuit or disconnection of the lights and lamps, and to inform the driver of the vehicle of the abnormal conditions thus detected in order to maintain a safe running vehicle.
Conventionally, a method has been proposed in which a load current flowing through a switching element, e.g., power MOS-type transistor or the voltage V.sub.Ds between the drain-source of the transistor connected in series to the load so as to drive the load, is monitored in order to detect the short-circuited condition or disconnected condition of the load such as the lamps and the like (see, for instance, Japanese Patent Disclosures 139624/1983 and 222554/1983).
In the monitoring method according to the prior art, a low value resistor is connected in series to the MOS-type transistor and the voltage across the resistor is monitored in order to detect the load current flowing through the MOS-transistor and to detect the short-circuited condition. However, since the current has the same amplitude as that of the lamp driving current, e.g., the current of about a few amperes flows through the resistor always when driving the lamps, there was a problem that the outer shape of the resistor and the power consumption became large.
Furthermore, when a rush current flows through the load such as lamps at the time of turning on a power supply, it is normal that a short circuit current during a time period corresponding to the rush current, e.g., a few tenths of msecs is not monitored in order to avoid mistaken detection of the rush current as the short circuit current. Accordingly, during at least the time period described above, a large thermal dissipation plate must be provided so as not to damage the MOS-transistor even when the current corresponding to the short circuit current flows through the MOS-transistor.
On the other hand, in the method for monitoring the voltage drop between the drain and source of the MOS-transistor to detect the disconnection of the lamps according to the prior art, since the voltage drop depends upon the on-time resistance of the MOS-transistor and upon the temperature characteristic, another correction circuit is required in order to correct errors due to the dispersion among the on-time resistances of the MOS-transistors and the change in the voltage characteristic of each of the MOS-transistors the loads are connected in parallel as in the case of brake lamps having two to four lamps in number. Accordingly, this has made the method according to the prior art uneconomical.